Additives comprising fractions of a percent to several percent of solid propellant mixtures have been considered through the years and are commonly employed in many rocket propellant and explosive compositions. Various additives include burn-rate modifiers (e.g., ferric oxide, metal oxides, and organometallics), curing agents, and plasticizers. In certain cases, additions of small (e.g., <5% by weight) amounts of powdered material to the propellant mixture have been shown to increase or otherwise favorably modify the burn rate. Nanoparticle additives may have an even further influence on the burn rate because of their high surface-to-volume ratios.
Powdered aluminum (metal) is commonly added to solid propellant formulations to increase energy density and specific impulse. Ultrafine or nanoparticulate aluminum has the additional potential to increase burning rates due to its much higher surface-to-volume ratio. However, these materials suffer from difficulties of manufacture, safety hazards, and increasing % aluminum oxide content as the particle size decreases.
Several routes have been developed to produce aluminum nanoparticles, including wire explosion “top-down” methods, and chemical synthesis “bottom-up” methods. Chemical synthesis is advantageous in that no specialized equipment besides standard inert-atmosphere chemical reactors is required, and the bottom-up approach provides the ability to precisely control and tailor the material properties, such as particle size and the passivating oxide layer.
AlH3 (or alane) is a sterically and electronically unsaturated moiety that reacts readily with a wide range of Lewis donors. Amine adducts of alane have emerged as convenient aluminum precursors due to the ability of these compounds to decompose to metallic aluminum in the presence of an organometallic titanium catalyst, including at room temperature. Several studies have investigated the use of different solvents, catalysts, temperatures, and organic/inorganic passivating agents. However, this reaction has proven difficult to control due to the fast reaction kinetics and sensitivity to precise reaction conditions. Additionally, these procedures produce agglomerates of aluminum nanoparticles, which precipitate from solution as powders. Besides being a safety hazard, these aluminum powders are difficult to effectively disperse in the polymer binder of the propellant. The aluminum particle agglomeration reduces the ultimate potential of nano-aluminum to provide large increases in propellant burning rates.